Apple has rejected claims that it is unfairly blocking rival companies from putting their software onto the iPhone, as US regulators probe whether the company and its partners acted illegally by obstructing Silicon Valley rival Google.

The informal investigation by the Federal Communications Commission (FCC) surrounds the fate of Google Voice, a free application that allows users to direct their phone calls and send text messages via the internet.

Google Voice was rejected from Apple's iPhone app store last month, prompting speculation that it was blocked because it competed with – claims that led the FCC to begin looking into whether the move constituted anti-competitive behaviour.

But in public statements to the commission on Friday, both Apple and AT&T - America's largest telecoms provider and the country's exclusive iPhone supplier – downplayed the way the situation had played out.

In a letter to the FCC which it published online, Apple denied that it had rejected Google Voice and instead said it had simply not yet approved it. According to the company, it had not given the program the go-ahead because it

"It appears to alter the iPhone's distinctive user experience by replacing the iPhone's core mobile telephone functionality and Apple user interface with its own user interface," said the company.

"Apple spent a lot of time and effort developing this distinct and innovative way to seamlessly deliver core functionality."

Although early speculation had suggested that the move was brought at the bidding of AT&T, amid possible concerns that Google Voice's free services could undermine its business, the company denied that it had any involvement in the decision.

"To that end, let me state unequivocally, AT&T had no role in any decision by Apple to not accept the Google Voice application for inclusion in the Apple App Store," he said. "AT&T was not asked about the matter by Apple at any time, nor did we offer any view one way or the other."

Google, which filed its own response to the commission's questions, did not reveal publicly what reasons Apple had given it.

The investigation is the FCC's latest attempt to tackle in America's mobile phone industry, but it also marks the first time that Apple has spoken openly about the process of reviewing iPhone applications – a procedure that has caused problems for many programmers in recent months thanks to its high level of secrecy and seemingly arbitrary standards.

In its letter, Apple revealed a number of details that it has so far refused to share with the plethora of application developers that it subjects to the process. The company revealed that it has 40 full-time reviewers who examine more than 8,000 applications each week, and that each application is checked by two individuals to ensure "that the review process is applied uniformly".

As well as raising questions about the nature of America's mobile industry, however, the investigation also marks the latest stage in the growing rift between Apple and Google.

In the past, the two companies had been very close, with Google chief executive Eric Schmidt even taking a seat on Apple's board in 2006. But after pressure over the role, particularly since the companies compete in an increasing numbers an increasing number of areas such as mobile phones, web browsers and now computer operating systems, Schmidt left the job earlier this month.